Ultrasound response to time-reversal symmetry breaking below the superconducting phase transition

Abstract: Ultrasound attenuation is a powerful probe of symmetry-breaking phenomena in superconductors. In this work, we develop a framework to model the ultrasound response of multi-component superconductors undergoing a time-reversal symmetry breaking transition below the superconducting phase transition. By coupling the elastic strain of the crystal lattice to the superconducting order parameters through group-theoretical analysis of tetragonal crystals, we classify how different symmetry channels contribute to the ultrasound signal. Using a two-component Ginzburg--Landau theory, we analyze the temperature dependence of sound velocity across both superconducting and time-reversal symmetry breaking transitions for several cases, including $(A_{1g}, A_{1g})$, $(A_{2g}, B_{1g})$, and $E_g$ representations. Our results demonstrate that ultrasound measurements are highly sensitive to the presence of bilinear Josephson couplings and can distinguish between different realizations of the superconducting state. We further show how external strain can significantly alter the ultrasound response in systems breaking time reversal symmetry.